The design of the coaxial interfaces and coaxial transmission lines is a well understood art. For a given impedance, the geometry of a differential transmission line is readily calculated using a two dimensional field solution algorithm. When making measurements of differential devices, the measurement instruments typically have 50 Ohm coaxial interfaces appropriate for the frequency band of interest.
However, most calibration procedures make measurements at single-ended ports, whether using PCB probes or some other measurement device. When the differential device is measured, the conversion from single-ended to differential propagation takes place at the transition from the calibration ports to the differential device under test (DUT). Any imbalance or mode conversion that takes place at this transition becomes a part of the measurement itself. Thus, there are advantages of making the transition within an adapter or connector to present a differential interface to the DUT.
Moreover, in order to more accurately measure differential devices, it is desirable to convert from the coaxial instrument ports to the differential interface geometry of the DUT before attachment to the DUT in a way that includes the conversion as part of the calibration procedure.
Thus, it would be advantageous to have an adapter or connector that provides a connection between coaxial to differential environments and compensate for discontinuities in impedance at the transition.